1. Field of the Invention
The present invention relates to an active noise control apparatus, and particularly to an active noise cancelling apparatus which actively cancels periodic noise generated from a rotation drive portion disposed in a machine chamber, by means of outputting a control signal of opposite phase but same amplitude as the noise signal.
2. Description of the Prior Art
A refrigerator at home and an air-conditioning equipment in a building are used continuously regardless of seasons, and noise therefrom is a problem. In this case, the troublesome noise source is a machine chamber which stores a rotary machine such as a motor.
To cope with the problem of the noise from the machine chamber, the conventional techniques include reducing the noise of the rotary machine itself, providing sound absorbing and insulating members within the machine chamber, and improving the noise absorbing level in the machine chamber and sound transmission loss.
However, there opening portions are provided for radiating heat caused by the rotary machine in the machine chamber, and thus the noise leaks outside. Thus there are limitations in conventional noise prevention techniques, particularly in reducing the noise level at the low-frequency band.
Recently, along with technical advances in electronics-applied technology, especially processing circuits for acoustic data and acoustic control, attention is being directed to an active control technique in which reduction of noise is attempted by utilizing interference of sound waves. In the active control technique, sound from a sound source is detected by sound source detecting means such as a microphone provided in a specific position and the sound detected is converted to an electric signal. The electric signal is processed by a computing element, so that an artificial sound having an opposite phase but same amplitude than that from the sound source at a control point is produced to attenuate the noise by interfering the artificial sound with the noise. The artificial sound is outputted from control sound outputting means such as a loudspeaker.
Namely, in the active control technique, the microphone is provided near the rotary machine of noise source, and the sound caused by driving the rotary machine is detected by the microphone. The electric signal which is processed by the computing element so as to damp the detected sound is outputted by the loud speaker so that both sounds are interfered attenuating the noise which is to be emitted outside.
An adaptive-type active control technique is also available where a noise-cancelling level at a noise cancelling point according to noise cancelling effect responsive to time lapse and change in sound is detected by a sensor connected to a control-sound generating filter in a feedback manner so as to maximize the noise cancelling effect.
The low-frequency noise which is controversial nowadays has a long wavelength as sound, thereby being apt to permeate the sound absorbing members and diffract an obstacle, so that there is not much expected in terms of noise preventing techniques such as using a noise shielding member or sound absorbing member. In contrast, the active control technique is effective at a low frequency.
FIG. 1 shows an example of such active control system. There are arranged a noise source 5 in an end of a space 3 within a duct 1, and an opening portion 7 in other end. There is provided a noise cancelling system 9 for cancelling noise generated by the noise source 5. In the noise cancelling system 9, there are provided a microphone 11, at point Ps of the duct 1, for detecting noise generated by the noise source 5, a control portion 13 for processing a signal detected by the microphone 11 so that a sound pressure thereof is zero at point Po near the opening portion 7 by sound wave interference, and a loudspeaker 15, mounted to the duct 1, for generating a control sound in the space 3. Thereby, a sound wall is formed at point Po which becomes a noise cancelling point, so that the noise is confined inside the duct without being radiated outside and the noise is cancelled.
A microphone 17 provided at point Po serves to detect the noise which remained uncancelled (not cancelled even after the above noise cancelling process) and the microphone 17 is also needed for obtaining a filter processing characteristics at the control portion 13. In order to form a signal for cancelling the noise at the control portion 13, it is necessary to measure in advance the acoustic characteristics of the duct 1, the microphone 11 and the microphone 17 and to obtain the characteristics for the filter which processes the sound source signal based on the acoustic characteristics in the control portion 13. A method for obtaining such characteristics is described as follows.
First, when the loudspeaker 15 generate a random noise, an acoustic transfer function Gao (referred to as simply the transfer function hereinafter), including the characteristics of the loudspeaker, between points Pa and Po is measured. Second, while the random noise is being generated from the loudspeaker 15, transfer function Gso between points Ps and Po is measured. Then a signal detected at point Ps is processed. Let a transfer function which represents up to the point where the control sound is generated at point Pa be Gsa. Gsa is an acoustic transfer function between points Ps and Pa. There is a relation such that: EQU Gso=Gsa.multidot.Gao (1)
Thus, transfer function G for the control portion 13 is one which a phase which opposite to the phase of Gsa and G is obtained by: EQU G=-Gsa=-Gso/Go (2)
On the other hand, in order to maintain great noise cancelling effect in the course of forming the control sound, there is necessitated a function for automatic control which takes into account the time-lapse changes in the microphones 11, 17 and the loudspeaker 15 as well as changes in the acoustic function found in the space 3 responsive to a change in temperature and so on. Thus, the adaptive-type active control system is proposed therefor.
Referring to FIG. 2, in the adaptive-type active noise cancelling system, there is provided a sensor (microphone) at the noise cancelling point, through which the uncancelled noise is constantly monitored and fedback to the control portion so that a monitor signal thereof is minimized. In FIG. 2, elements such as the duct, microphones and loudspeaker are omitted.
In the adaptive-type active control system, transfer function Go from the loudspeaker to the noise cancelling point is measured in advance in a similar manner as with FIG. 1, and transfer function Go is set in a factor setting portion 19. Let a sound signal from the sound source be Sx, and a sound signal at the opening portion of the duct be Sy, there is a relation such that: EQU Sy=Gso.multidot.Sx (3)
In order to cancel sound signal Sy at the opening portion, it suffices to overlap sound signal -Sy which is opposite in phase but with same-amplitude as sound signal Sy, over the sound signal Sy at the opening portion of the duct. Let Sa be a signal which is outputted to the loudspeaker as the control sound, then -Sy is expressed by: EQU -Sy=Go.multidot.Sa (4)
Moreover, referring to FIG. 2, let the characteristic of a filter 21 for cancelling noise, namely, transfer function thereof be G, then the control sound Sa is expressed by: EQU Sa=G.multidot.Sx=-Gso/Go.multidot.Sx (5)
Substitute equation (5) into equation (4), to obtain: EQU Sy=(-G).multidot.Gao.multidot.Sx (6)
Hence, as evident from equation (6), transfer function -G is obtained from Go-Sx where sound signal Sx from the sound source is filter-processed by transfer function Gao of the factor setting portion 19. Then, the characteristics of the filter 21 for cancelling the noise is obtained by inverting the sign of the transfer function -G.
When the above-mentioned process is carried out by a digital filter instead, the characteristics of the filter for cancelling the noise is obtained as a filter factor, so that an inversion of the factor sign is obtained by subtracting each tap factor value from zero.
Moreover, when transfer function Gso is dislocated to Gsoa and an optimum value of characteristics for the noise cancelling filter is dislocated by .DELTA.G to become Gnew from Gold, where Gnew=Gold-.DELTA.G (7), Sya which is a signal uncancelled at the opening portion of the duct is expressed by: EQU Sya=Sx.multidot.G.multidot.Gao+Sx.multidot.Gsoa (8)
Hence, there is shown a relation at an optimum noise cancelling condition: EQU Sx.multidot.(G-.DELTA.G).multidot.Gao+Sx.multidot.Gsoa=0 (9)
Eliminating Gsoa in equations (8) and (9), ##EQU1##
Gsoa is an acoustic transfer function between points Ps and Po whenever Gso is changed, as described below.
Hence, in the similar manner as with equation (6), in an adaptive filter 23 a dislocated component (.DELTA.G) of transfer function is obtained from Go.multidot.Sx where sound signal Sx from the sound source is filter-processed by transfer function Gao of the factor setting portion 19, and Sya which is the uncancelled sound signal in the opening portion of the duct. Dislocated component .DELTA.G is sent from the adaptive filter 23 to the noise cancelling filter 21, and Gnew representing the optimum value for the characteristics of the noise cancelling filter 21 can be obtained from equation (7).
Here, comparing equation (6) with equations (7) and (10), the initially obtained characteristics G for the noise cancelling filter 21 is, in equation (7), equivalent to: EQU Gold=0 (11)
A process for cancelling noise can be shifted toward an optimum condition by repeating a process represented by equation (10) with an initial value for the characteristics of the noise cancelling filter being 0, and the factor-updating process represented by equation (7).
In reality, it is advantageous to adopt the following equation where feedback gain parameter A is multiplied by .DELTA.G so as to improve the converging rate and stability: EQU Gnew=Gold-.mu..multidot..DELTA.G (12)
However, in the above adaptive-type active control there are several problems as to practical use thereof, as explained below.
In detecting the noise from the sound source directly by the microphone and so on, howling may occur when not only noise of the rotary machine but also the control sound outputted form the loudspeaker are picked up by the microphone. In this case, the howling offsets the noise cancelling effect and noise cancelling effect is no longer available.
To solve such a problem, a vibration pickup sensor is provided for detecting vibration of the rotation drive portion (noise source) in order to detect only the rotary machine which is the sole sound source. Namely, the vibration pickup sensor comprising piezoelectric elements, etc. is directly mounted on the rotary machine, so that the noise generated from the rotary machine alone is detected and a noise-cancelling signal based on thus detected noise is generated, thereby cancelling noises without causing the howling to occur.
However, there are several disadvantages caused by employing the vibration pickup sensor which is directly mounted to the rotary machine, described as follows.
Since the rotary machine generates heat as usage thereof continues, it is necessary to use the pickup sensor which is heat-resistant against a high temperature, thus causing an increase of cost in designing and producing such heat-resistant pick up sensor. Moreover, in using the pickup sensor, there is, in general, a charge amplifier is used for amplifying the detection signal. However, since it is difficult to mount the charge amplifier near the rotary machine, the charge amplifier will have to be provided separately from the pickup sensor connected by a cable, so that a weak signal detected by the pickup sensor is affected by an unwanted electric noise.
In recent times, big refrigerators are desirable. Consequently, the size of the compressor in a refrigerator becomes bigger, thereby causing an increase in the noise and heat generated by the compressor. In order to cope with such large-sized compressors presenting the increased noise and heat, a plurality of opening portions have to be provided in the machine chamber where the compressor is housed.
As a result, in order to cancel noise of the compressor of the large-sized refrigerator, where there a plurality of radiating opening portions are provided, there is not enough noise cancelling capacity in a conventional active noise cancelling system which is primarily designed for the machine chamber with a single opening portion. Further, it will be costly to mount the conventional noise cancelling apparatus in a plurality of opening portions in the limited space provided.